The invention relates to a display device including a display tube whose neck accommodates an electron gun system for emitting at least one electron beam toward a facing display screen, and an electromagnetic deflection unit arranged around the envelope of the display tube. A first deflection coil and a second deflection coil arranged coaxially with respect to the first deflection coil each have a front end facing the display screen and a rear end, the deflection fields generated by the two deflection coils upon energization being at right angles to each other and both extending in a part of the space between the gun system and the display screen.
In monochrome display tubes the electron gun system is adapted to produce one electron beam. In colour display tubes the electron gun system is adapted to produce three electron beams.
In monochrome display tubes for, for example data display uses and for t.v. projection tubes the aim is to use a deflection unit with deflection coils which give such a field distribution that the spot quality is as perfect as possible both in the centre of the display screen and in its corners.
For some time colour display tubes have been used in which three spatially separated electron guns are located in one line. Such a display tube is known as an in-line colour display tube. In the in-line colour display tube the aim is to use a deflection unit with deflection coils giving such an inhomogeneous field distribution that the beams of the electron guns coincide across the entire screen when they are deflected. To this end particularly the horizontal deflection field (to be generated by the second deflection coil) must be barrel-shaped on the gun side of the deflection yoke and pincushion-shaped towards the screen side and, conversely, the vertical deflection field (to be generated by the first deflection coil) must be pincushion-shaped on the gun side and barrel-shaped towards the screen side.
The extent of pincushion and barrel shape is such that in the case of deflection the convergence errors of the electron beams emitted by the electron guns are corrected so that pictures having satisfactory convergence properties can be produced on the screen of the display tube. Combinations of display tube and deflection yoke of this type are referred to as self-convergent.
In the design of deflection units for monochrome data graphic display tubes the starting point has so far always been that long (dipole)deflection fields generate the fewest astigmatic effects. For example, European Patent Specification No. 53 853 is based on this recognition and it discloses the use of horizontal and vertical deflection fields of equal, though larger length than is conventional practice, the identical sixpole field components (first a positive component and then a negative component viewed in the direction from the display screen to the gun). However, also in deflection coils which are designed and manufactured very carefully on the basis of this recognition the effect occurs that the electron beam spot is perfect in the corners of the display screen, but is not quite perfect at the ends of the screen axes. Improvement of the spot quality on these axes by means of a different distribution of the sixpole field components results in a deterioration of the spot quality in the corners.